There are many fields where operating a motor at a desired speed with little or no speed variation due to torque disturbance is required. For example, engine characterization requires operation at a selected speed. In a cold engine test system, an electronic motor drive is used to rotate an engine under test at a desired speed for purposes of, for example, engine design optimization, engine cylinder leakage characterization, etc. However, the engine under test exerts undesirable periodic torque disturbances on the motor shaft due to the engine cylinder compression/cam linkage interaction. The periodic torque disturbances are harmonically related to the rotational speed of the engine cam shaft and crank shaft and cause the actual engine speed to vary, which is not desirable for the characterization being done. Other areas that may produce periodic mechanical torque disturbances include rolling mills, rotary and reciprocal pumps, coilers and uncoilers, etc. Periodic torque disturbances may also occur due to the electrical distortion caused in power electronic driven motor drives resulting from, for example, the dead time between phase leg switching events.
Several techniques have been developed to reduce the effects of periodic torque disturbance. Raising the bandwidth of the drive speed loop can lower the resulting engine speed variation, but does not remove it entirely. Another technique adds a disturbance torque observer to the speed loop to decouple and minimize the speed variation. However, error of the acceleration estimate term of the observer often leads to non-ideal decoupling and speed variation.
Another technique is referred to in literature as a repetitive controller. This technique has several drawbacks. One drawback is that it does not learn or compensate for the phase of each harmonic of interest. The repetitive controller has infinite gain (i.e., integral action) at every multiple of the harmonic of interest and “learns” the magnitude. The same amplitude of correction is applied to the harmonic of interest and each of its multiples. In an actual system, each multiple of a harmonic may require a different amplitude for its compensation. In order for the repetitive controller to work properly, the compensation for harmonic multiples not of interest must be removed. One method is by performing a Fast Fourier Transform (FFT), removing the bins containing the multiples not of interest and performing an inverse FFT, which is a cumbersome process. Additionally, the repetitive controller in many instances becomes unstable, which results in online learning of harmonics being precluded.
Another application for harmonic regulation is one where the harmonic torque disturbance is deliberately introduced to the system. Such applications include, for example, test stands where the electric motor must simulate the torque pulsations inherent in an internal combustion engine for the purpose of testing transmissions, alternators, air conditioners, pumps and other equipment. The usual method has been to use a torque profile which is mapped to the position of the simulated engine crank. This method has the disadvantage of not having good control at higher frequencies as a result of the limited bandwidth of the basic control algorithm.